A smart grid is a next generation smart electrical grid by which a conventional energy provider and consumers can communicate with each other in bidirection instead of unidirection. It can utilize energy generated from environment-friendly new renewable energy sources, such as sunlight, wind power and tidal power, as well as base energy sources, such as atomic power and thermal power. Also, the smart grid requires an energy storage device for storing energy input from various energy sources in order to perform two-way communication. In general, the energy storage device includes a secondary battery pack having multiple secondary battery cells and module.
Meanwhile, in a smart grid environment, an energy storage system for a micro smart grid is required, which can maximize energy efficiency by efficiently storing energy input from various energy sources in an energy storage device, managing and maintaining the stored energy, and controlling and managing the energy so as to effectively distribute power of a power source according to load condition of an output load at the proper moment. Further, a wireless energy transfer technology of wirelessly storing energy in an energy storage device, which has no wires and thus is simple and can be utilized in various application fields, has been recently gradually considered more importantly than a wired energy transfer technology of wiredly storing energy in an energy storage device.